We have developed a sensitive ELISA assay for measuring the potency of trivalent poliovirus vaccines using mouse monoclonal antibodies for antigen detection. During routine tests of IPV we found a vaccine lot the type 2 component of which tested very low with our standard reagent; but gave satisfactory values with another type 2 monoclonal antibody or rabbit polyclonal sera. We are collaborating with another laboratory as a separate research project to identify the site specificity of our monoclonal antibodies. We are in the process of trying to elucidate the cause for this apparent epitope specific change in the type 2 component of lots of vaccine which passed in the ELISA test. We reviewed the literature for chemicals known to affect the potency of poliovirus. We are examining the effect of thimerosal and other combinations of preservatives eg. EDTA, 2-phenoxyethanol and formaldehyde on the potency of IPV. Preparations have been held at three temperatures 37, 25 and 40C for 2 weeks. The preparations were tested for potency by ELISA at 2, 10, and 14 days. Vaccine preparations held at 37 and 40C were inoculated into mice on day 14. By looking at the potency of the vaccines in vivo we will have a better understanding of whether the drop in potency detected by ELISA is reflected in a lowered immunogenicity in mice. In addition, we are looking at the effect on potency of mixing vaccines containing thimerosal with IPV. These findings will have important implications for manufacturers of combination vaccines as they will need to consider the effect of each preservative on the antigens in the combination.